Millions of bicycles and the like have been sold with variable speed transmissions that are manually shifted, but efforts to make an automatic shifting device have not been so successful on a commercial scale.
The reasons why a cyclist should want an automatic shifting device have been reviewed by R. E. Perry in U.S. Pat. No. 3,929,025.
Known designs for automatic shift controls are intended to be responsive either to speed changes or to force changes. U.S. Pat. Nos. 3,301,095 and 3,388,617 describe speed responsive devices with centrifugal weights. The U.S. Pat. No. 3,929,025 describes a force responsive device, working with chain tension variations, and having mechanical means to prevent excessively rapid shifting due to cyclic pedal positions or shock loads. U.S. Pat. No. 3,613,466 is related.
It is important to recognize that there are several kinds of variable speed transmission systems widely used for bicycles and similar pedal-powered vehicles. Such variable speed transmission systems may be called adjustable speed force transmission systems, and they commonly are found connected to the rear hub of a two wheel bicycle. The great majority of these are manually shifted by a mechanical linkage that delivers an important push or pull action to some shiftable element in the transmission system. We are concerned here with the system that initiates a shift and delivers the important push or pull action to the shiftable element. Accordingly, the automatic shift control can be applied to any pedal-powered vehicle having either a derailleur type transmission, a multiple speed hub, or some other type of shiftable transmission, including infinitely variable types.